The present invention relates to a method for forming a dot element, a semiconductor device using the dot element and a method for fabricating the device. More particularly, the present invention relates to a method for forming a dot element out of an ultrafine particle of the size of several nanometers and functioning as a quantum dot element, a semiconductor device using the dot element and a method for fabricating the device.
Currently, a ULSI is formed by integrating a great number of MOS devices on a single chip. In general, as an MOS device is miniaturized, the performance thereof is enhanced correspondingly. However, if the gate length thereof is 0.1 .mu.m or less, then the device can hardly operate normally as a transistor, because such a size is a physical limit for the device. A single-electron tunneling device, called a "coulomb blockade", has attracted much attention recently as a candidate for breaking through such a limit (Kenji Taniguchi et al., FED Journal, Vol. 6, No. 2, 1995). In principle, a single-electron tunneling device performs logical operations and storing operations by controlling the movement of individual electrons, and is very effective in reducing power consumption. However, in order to form a single-electron tunneling device, semiconductor or metal fine particles of the size of several nanometers, called "quantum dot elements", are required. As disclosed in Japanese Laid-Open Publication No. 9-69630, for example, if a large number of Au dot elements are formed out of Au fine particles by sputtering or the like between metal electrodes formed on a substrate, then the Au dot elements form multiple bonds with each other, thereby realizing single-electron effects. In accordance with this method, however, it is very difficult to accurately control the positions where the Au dot elements are formed.
Thus, Sato et al. proposed another method for forming a dot element. In accordance with the method of Sato et al., 3-(2-aminoethylamino)propyltrimethoxy silane (APTS) is deposited on a substrate on which a PMMA resist pattern has been formed. Then, APTS on the PMMA resist is partially lifted off together with an unnecessary portion of the PMMA resist, thereby selectively leaving APTS at desired positions on the substrate. Thereafter, Au fine particles are attached onto only APTS, thereby forming Au dot elements.
Aside from the single-electron tunneling device, a different method for breaking through the limit of a device size using dot elements was also proposed. For example, S. Tiwari et al. disclosed in IEDEM Tech. Digest, 521 (1995) that an operating voltage would be lowered by using dot elements of silicon fine particles for the floating gate of a nonvolatile memory or the like. Tiwari et al. suggested that silicon dot elements could be formed directly on a substrate by performing a CVD process on accurately controlled conditions.
However, the methods of T. Sato et al. and Tiwari et al. have the following problems.
To control the positions of dot elements on a substrate by the method of T. Sato et al., the process steps of forming a PMMA resist pattern or the like on the substrate and then lifting off APTS with unnecessary portions of the PMMA resist pattern are required. Thus, the fabrication process is adversely complicated. In addition, in this method, the Au dot elements are formed onto APTS by utilizing the polarization of charges. Accordingly, if charges have been polarized at other sites on the semiconductor substrate, then Au fine particles are unintentionally attached to such sites. Therefore, it is not always possible to selectively form the Au dot elements only at desired sites.
On the other hand, in accordance with the method of Tiwari et al., silicon dot elements are directly formed on a substrate by a CVD technique. Thus, it is very difficult to control the sizes and positions of such dot elements on the substrate.
Because of these inconveniences, it is now hard to use dot elements, formed by the conventional methods, as a member of a semiconductor device or as quantum dot elements, in particular. That is to say, in accordance with the conventional methods, a semiconductor device, including dot elements formed with the sizes and positions thereof accurately controlled, is very much less likely to be realized.